The present invention relates to an optical communication system and, more particularly, to the utilization of a phase-sensitive detection arrangement on each sideband of a double sideband signal to measure polarization mode dispersion (PMD).
Polarization mode dispersion (PMD) occurs in an optical fiber as a result of a small residual birefringence that is introduced in the fiber core by asymmetric internal stress or strain, as well as random polarization coupling due to external forces acting upon the fiber. Consequently, PMD may severely impair the transmission of a signal in an optical fiber-based network. There will be two polarization modes supported by a single mode transmission fiber. There is a group delay between these two eigen-modes, also knows as the principal states of polarization (PSP). If the input polarization is aligned with one of the PSPs, then the output polarization will remain in the same PSP. However, for arbitrary input polarizations, the output will consist of both PSPs, with a certain amount of delay (in time) between them. It is this differential group delay (DGD) that causes waveform distortion. In order to compensate for PMD, it is necessary to find the PSPs at the output so that a polarization splitter can be used to separate the two PSPS.
In the prior art, there are three basic categories of techniques used for polarization mode dispersion (PMD) compensation: (1) all-optical; (2) all-electrical; and (3) hybrid optical-electrical. For all-optical PMD compensation, the restoration of PMD distortion is performed optically and usually consists of a polarization converter coupled to a section of polarization maintaining fiber, or to a combination of a polarization converter, polarization beam splitter, a fixed and variable delay element and a combiner. The goal is to find the PSPs and align their axes to those of the PBSs. However, this is difficult to achieve since the principal states of polarization and the differential group delay (DGD) are not directly measured, and any optimization algorithm that is used to set the polarization converter and the variable delay element may converge to a local optimum or even fail to converge at all.
In a conventional all-electrical method, the distorted optical signal is first converted to an electrical signal at the receiver. A delay line filter with specific weights is then used to partially compensate for the distortion due to PMD. An exemplary hybrid technique may utilize a polarization controller and a section of polarization maintaining fiber. A high-speed photo-detector is used to convert the optical signal into an electrical representation. An electrical tapped delay line filter is then used to adjust the frequency-dependent phase of the electrical signal.
The widely-used modulation phase shift technique, as discussed in the article xe2x80x9cPhase Shift Technique for the measurement of chromatic dispersion in optical fibers using LED""sxe2x80x9d, by B. Costa et al., IEEE Journal of Quantum Electronics, Vol. 18, No. 10, pp. 1509-15 (1982), utilizes a double-sideband modulated signal and a swept carrier frequency to measure delay. The sidebands are required to exhibit the same amplitude and the change in delay and frequency must be accurately measured to obtain the dispersion parameter.
A need remaining in the prior art is addressed by the present invention, which relates to an optical communication system and, more particularly, to the utilization of a phase-sensitive detection arrangement on each sideband of a double sideband signal to measure polarization mode dispersion (PMD).
In accordance with the present invention, single-sideband self-homodyne signals are generated (or detected) and each sideband is separately processed to determine delay and dispersion information. By including a polarization beam splitter prior to the single sideband recovery, information on both principal states of polarization can be collected and then used to determine the polarization mode dispersion.
A narrowband optical filter is used to generate a signal including upper and lower sideband components from the received (and polarized) optical signal. The output form the narrowband filter is then used as an input to a phase-sensitive single sideband detector circuit which includes an RF signal generator, multipliers and phase shifter to form both in-phase and quadrature outputs. The magnitudes and phase of the output are used to form the dispersion information. By performing the detection on both polarizations, therefore, polarization mode dispersion information is obtained.
In a preferred embodiment, the narrowband optical filter includes a half-wave plate at its input and is used to process both polarizations of the optical signal, eliminating any discrepancies in the processed magnitude and phase information for each polarization.
Various and other embodiments of the present invention will become apparent, during the course of the following discussion and by reference to the accompanying drawings.